With plug&paid Stripe subscriptions you have the flexibility to charge your customers a signup (one-time) fee for subscribing to your service. To enable this feature scroll to find the "One-Time Fee" option in the "Add Product" screen. 

The fee is considered an additional "service/product" on the order and show as such in the invoice that is generated upon a successful purchase. The fee is always paid once as soon as the customer subscribes to your product. The same applies if you choose to add a Trial to your subscription. If you have taxes enabled then they will also be applied to the fee.